Abstract Unlike aliphatic mercaptanes, thiophenol and [Cu(CH 3CN) 4]ClO 4 form crystalline complexes of the type [Cu 4S 6]X 2-. Me 4N +, Et 3NH + or Bu 4N + served as the respective suitable cation X. The copper to sulphur ratio was close to 1:1.5. A comparison of the IR spectra of the free and the complexed ligand clearly demonstrates the disappearance of the characteristics ν(SH) vibration at 2570 cm −1 and supports the copper thiolate binding. From both the integration of the 1H NMR spectra and the elemental analyses a metal to ligand ratio of 1:1.5 has been found. The Cu 4S 6 cluster would nicely fit an adamantane-type structure as earlier described  (Fig. 1). Cu-thionein from baker's yeast has a [Cu(SR) 2] 4 centre. According to EXAFS spectroscopy  each copper is tetrahedrally surrounded by four cysteine sulphurs. The best guess for arranging four Cu(SR) 2− units was a cubane type structure (Fig. 2). At present the above mentioned low molecular weight [Cu 4S 6] species are the closest models for the Cu-thionein metal binding centre. Of course a ratio of 1 Cu per 2 thiolate sulphurs would most successfully mimic the copper binding in this protein.